Changing values inside Python Modules

Question

I have a simple Python module file First.py:

a = 50
b = [100,200,300]

I try to import this module into another file Test.py:

import First
First.a = 420
First.b[0] = 420
print (First.a)

My purpose is to change the list values inside the First module.

Once the script Test.py completes when I print the values inside the module , I find that the values have not changed.

Python 3.5.2 (v3.5.2:4def2a2901a5, Jun 25 2016, 22:01:18) [MSC v.1900 32 bit (I
tel)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> import First
>>> dir(First)
['__builtins__', '__cached__', '__doc__', '__file__', '__loader__', '__name__',
'__package__', '__spec__', 'a', 'b']
>>> First.a
50
>>> First.b
[100, 200, 300]

What am I missing here? Can someone kindly guide me?

Any changes are in the runtime, they are not persistent between sessions. The text of the module is not changed. — AChampion
– AChampion, Commented Nov 9, 2016 at 12:45

Dimitris Fasarakis Hilliard · Accepted Answer · 2016-11-09 12:58:55Z

1

Once the script Test.py completes when I print the values inside the module , i find that the values have not changed.

If you executed python Test.py, then fired up your interpreter interactively and checked the values, of course, modifications won't be visible. Python just loads First.py when the import is found, executing it and initializing a and b with the values in First.py; previous executions won't affect this.

If you import Test in your interactive interpreter and then import First changes will be reflected:

Python 3.5.2 |Anaconda 4.2.0 (64-bit)| (default, Jul  2 2016, 17:53:06) 
[GCC 4.4.7 20120313 (Red Hat 4.4.7-1)] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import Test
420
>>> import First
>>> First.a
420
>>> First.b
[420, 200, 300]

During the import of Test, First was loaded and executed, then its values modified. When you re-import python will just look in a table of imported modules (sys.modules) and return it without executing its content (and re-initializing a and b)

edited Nov 9, 2016 at 12:58

answered Nov 9, 2016 at 12:46

Dimitris Fasarakis Hilliard

162k35 gold badges282 silver badges265 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Subhayan Bhattacharya Over a year ago

But i am not sure i follow 100%. According to my understanding Python does not reload and run a module file once it is already loaded and .pyc file is created. So why in the first case the module code is re-run which ultimately reinitialize the variables and the code is not run the second time. In both the cases the module is already loaded.

Dimitris Fasarakis Hilliard Over a year ago

@SubhayanBhattacharya Yes, .pyc files are generated but that makes no difference. When you run (not import) Test.py changes it made to First.py aren't persistent; the .pyc file has the same bytecode instructions. When you import Test you also import First and then change its value, the .pyc doesn't really play a role here :-).

Collectives™ on Stack Overflow

Changing values inside Python Modules

1 Answer 1

2 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

2 Comments

Your Answer

Sign up or log in

Post as a guest

Related